The consequence of an over-rev
Discussion
My 986 has been off the road now for almost 4 years (link to original thread http://www.pistonheads.com/gassing/topic.asp?t=123... )
Well, the fault has finally been diagnosed through stripping the engine. A bent exhaust valve, broken spring and broken tappet with a piston that has hit the valve. Likely to be caused by an over rev which is a bit unfortunate (will be plugged in to check for over revs when its back together). Here are a couple of photos of the damage. Just waiting to get the full cost breakdown for parts.
Hopefully won't be too long until it's back on the road.
Well, the fault has finally been diagnosed through stripping the engine. A bent exhaust valve, broken spring and broken tappet with a piston that has hit the valve. Likely to be caused by an over rev which is a bit unfortunate (will be plugged in to check for over revs when its back together). Here are a couple of photos of the damage. Just waiting to get the full cost breakdown for parts.
Hopefully won't be too long until it's back on the road.
You were mentioning about the coil packs were changed by Porsche, just before this failure. Did they do a major service on the car? If so, I would assume they should also check the ECU data and see if the car has been over-revved, so that they could have also suggested a preventative maintenance like your valves to be checked before the engine grounded itself!!
Is it me or OPC not really sensitive about these cars during both minor and major service schedules?
Is it me or OPC not really sensitive about these cars during both minor and major service schedules?
Is that all three cylinders on the same bank where the valves have kissed the pistons? Hasn't skipped a tooth on the cam drive, no?
If that damage is a result of over-rev, you have to expect the other bank to be the same. Given the mode of failure, constant throttle, relatively low engine rpm on the motorway (as per original thread) I would be looking for another cause or reason beyond over-rev. That looks like an instantaneous failure - i.e. the damage was done at the point you experienced running issues on the road as the witness marks are too fresh. That suggests something happened at the time you were driving to cause the failure, like slipped cam-timing. Further investigation required.
If that damage is a result of over-rev, you have to expect the other bank to be the same. Given the mode of failure, constant throttle, relatively low engine rpm on the motorway (as per original thread) I would be looking for another cause or reason beyond over-rev. That looks like an instantaneous failure - i.e. the damage was done at the point you experienced running issues on the road as the witness marks are too fresh. That suggests something happened at the time you were driving to cause the failure, like slipped cam-timing. Further investigation required.
Looks like exhaust valves on most cylinders have also touched the pistons, so would be a good idea to check they are not bent, even if it's a tiny bit. I was also wondering if one of the inlets has just touched on other cylinders, difficult to see from pictures, but thought I could see a slight witness mark.
Most likely cause is an over-rev. Manual box? Normally caused by early downshifts rather than anything else with modern engine management.
You can sometimes get away with the pistons (get them checked very carefully by a good engine builder before taking a chance), but slightest suspicion of bent valves, they really need replacing.
Hoping I'm not teaching my granny to suck eggs, but have been there many, many times before with all sorts of engines.
Liam
[just read your original thread. Over-rev seems very unlikely - something has possibly happened to the cam timing as Larry says. Needs careful analysis to find why it has happened, and a very careful check of all valves, tappets, camshafts and pistons.]
Most likely cause is an over-rev. Manual box? Normally caused by early downshifts rather than anything else with modern engine management.
You can sometimes get away with the pistons (get them checked very carefully by a good engine builder before taking a chance), but slightest suspicion of bent valves, they really need replacing.
Hoping I'm not teaching my granny to suck eggs, but have been there many, many times before with all sorts of engines.
Liam
[just read your original thread. Over-rev seems very unlikely - something has possibly happened to the cam timing as Larry says. Needs careful analysis to find why it has happened, and a very careful check of all valves, tappets, camshafts and pistons.]
Edited by LiamH66 on Friday 30th September 00:19
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